The invention relates to a commutator motor such as a series motor or a permanent magnet motor to be incorporated in portable machine tools, preferably hand machine tools such as drills, the motor comprising a stator with a winding or a permanent magnet arranged in the machine housing and a rotor with a commutator journalled therein, the electric connection between the commutator and current supply being ensured by a pair of diametrically opposing brushes having different polarities.
The invention has for its object to improve the electric motor defined in the preamble in a sense such that its properties can be adapted to the job to be carried out by means of the machine. For example, the sense of rotation of the electric motor can be electrically inverted without the need for a mechanical inverting device, such as a gear box.
The motor embodying the invention is distinguished in that the brushes are supported in the machine housing so as to be displaceable with respect to the commutator axis for varying the field angle between the stator and rotor. In this way the optimum field angle can be set so that the motor can match the desired operation. In conventional machine tools the optimum field angle depends on various factors such as geometry of the stator, nominal speed or rotation, rotating mass and materials employed, while at the optimum field angle determined by these factors, an anti-interference circuit ensures that during rotation of the motor the electro-magnetic interferences are held within acceptable or required limits, while the wear of the brushes remains slight. In a further development, the invention proposes two additional pairs of carbon brushes of different polarities, which pairs can be separately brought into contact with the commutator by a setting means. In this way fixed field angles can be set, as a result of which not only brush wear is appreciably reduced, but also the desired anti-interference circuitry can be provided in a simple manner.
When using such a brush system serving to invert the sense of rotation of the motor, only two pairs of carbon brushes need be employed, which have the same field angle with respect to a neutral line so that one and the same anti-interference circuit is sufficient.
In a simple embodiment, it is preferred to arrange the brushes of the pairs intended for the same polarity on a common carrier with a common current conductor.
The setting means may be formed by an element being rotatable concentrically with the commutator axis and having a handle actuable externally of the machine housing, the element being provided with cams for lifting the brushes but one having the same polarity, while the brushes can be pressed by spring tension against the commutator ring.
As an alternative, the setting means may be designed so that they bring only one pair of brushes of different polarities into contact with the commutator ring.
Further, the invention relates to a portable machine tool having a machine housing, a handle provided with a switch for energizing an electric motor described above and arranged in the housing, the machine tool being distinguished in that the setting means are provided with a blocking member for blocking the switch in a position of the setting means in which the brushes of different polarities other than those located diammetrically opposite one another are in contact with the commutator.
Thanks to this blocking member, a potential unintentional switching or a shortcircuit is avoided.
Further, the invention relates to an electric motor mainly comprising a stator and rotor arranged in a housing provided with a commutation ring and brushes co-operating therewith, each of which is arranged in a holder which is pivotable with respect to the housing.
The invention has for its object to improve the suspension or fastening of the brushes in the electric motor in a manner such that the introduction of the brushes into the holder is facilitated, the electric contact between brush and holder is improved, the shape of the brush itself remains simple and/or the pressing force of the brush on the commutator ring can remain substantially constant even after some amount of wear and, respectively, can exhibit an increasing or decreasing pressure characteristic curve.